You mention the "scale" of game, but you seem mostly to mean the size of the game. I offer that distinction because it may help you narrow the field if you do actually determine what scale of game you want to play -- I mean what unit scale in this case.
Many rulesets that are popular with 6mm models are 1-to-platoon or 1-to-5 unit scales. In such cases one model tank on the table represents a platoon of tanks, or maybe 5 tanks, in the game. This approach seems to be the most popular these days.
Other rulesets to to even larger unit scales, with 1-to-company or even 1-to-battalion scales, allowing gamers to play brigade or even division sized games.
My own preference is 1-to-1 for vehicles. I prefer a game where one tank model represents 1 tank. For the infantry I prefer a squad level unit scale (using American vocabulary). I want one stand of infantry to generally represent a squad of 8 – 12 men (I think the equivalent in British Army would be section). There are rules that are 1-to-1 on vehicles that go down one level further, where one stand would equal what the US Army calls a fire team or a section (often confusing to those accustomed to Brit-speak), which is a sub-unit of the squad with 3 to 6 men.
My current preferred ruleset is Mein Panzer, published by Old Dominion GameWorks (ODGW here: odgw.com/home.html). They also publish the General Quarters 3rd Edition naval rules which are pretty well regarded, as well as rules for ACW and Napoleonic era warfare.
I find Mein Panzer to be the best rules I've played yet for my preferred type of game. I like combined arms, and I find this ruleset mixes tanks, infantry, support guns, etc. better than others. In particular I have seen too many games where the tanks are doing fine, then the infantry gets out of their transports and the game grinds to a halt, with the game ending after just one or two more crushingly long slow turns. Mein Panzer are reasonably fast play There's enough detail to keep me interested (so for example a T-34 has different armor ratings than a Pz IIIm), but not so much detail that it dominates the game (such as a T-34m41 having a weak spot at the hull MG mount that was improved on the T-34m42). And the infantry play at about the same speed as the tanks. Very balanced.
It is generally an I-go-You-go turn, but the game turn is divided into multiple activations. You don't have to make any roles to activate. Each activation you get to play one "unit". You chose which unit, and you activate it. Each unit activates only once per turn, so after each side has activated all their units the turn ends and you start on the next turn.
Units are generally platoon-sized formations (typically 3 to 5 stands, but might be only 1 or 2 stands). Could be actual platoons, could be an HQ, could be a battery of guns, it depends on your force. When you activate a unit you get to play only those tanks (or troops, or guns) in that unit. So as a gamer you are forced to think in terms of your force structure -- you don't just see a bunch of tanks scampering around, but you see platoons of tanks working together. But there is no paperwork or tables and die-rolls or other external overlays causing this -- it's just the way the turn works.
The infantry are generally one squad per stand. Most squads have a combination of weapons, often including an LMG. All of this is factored in to the squad's combat rating. But sometimes you also have smaller "team" stands -- for example a support weapon like a machine gun might be a separate team stand. Squad sized stands will be harder to eliminate than smaller team stands (which is right, as they have more men).
The rules have core chapters, and then there are optional add-on chapters (for things like field engineering, air support, amphibious operations, etc.). You can ignore the optional add-ons if you just want to get playing with tanks and infantry. Or you can build up to almost everything that ever appeared on a mid-20th century battlefield.
I find the rules play very fluidly. In my experience each player can run maybe 15 to 25 tanks / stands, and the game flows admirably well. Once players are quite familiar with the rules they might be able to manage 30 stands or even a few more. At one-to-one (and one-to-squad) unit scale that generally means a full company with one or two attachments per player (or maybe a battalion of tanks for a Russian player). The game behaves quite well when the number of players grows -- so battalion-sized games with each player running a re-enforced company are quite reasonable if you can gather enough guys around the table. As the turn is broken into small "activations", all players are involved almost all the time, rather than one side having to measure and move all of their stands while the other side goes off to get food, grow beards or write novels.
I've played many rulesets over the years (many years ago, mind, so not all of the current rules). I settled on these more than a decade ago and have not budged since.
Your tankage may vary.
-Mark
(aka: Mk 1)
